1. Field of the Invention
The present invention relates in general to Global Positioning System (GPS) signals, and in particular to a method and apparatus for analog compression of GPS clear/acquisition (C/A) signals to audio bandwidths.
2. Description of the Related Art
Currently, there exists a position determining system, referred to as the NAVSTAR Global Positioning System (GPS), wherein a constellation of 24 orbiting satellites transmit pseudo-random ranging signals from which users with appropriate equipment can obtain three dimensional location, velocity and timing information anywhere on or near the surface of the Earth. The longitude, latitude and altitude of any point close to Earth, with respect to the center of the Earth, will be calculated by determining propagation times of electromagnetic signals from one or more of the satellites to the point in question.
A signal about a single center frequency from one or more of the visible satellites will be received by a user terminal at a point close to Earth to measure propagation times of the electromagnetic signals transmitted by the satellites. The satellites from which the signals originate are identified by modulating the signal transmitted from each satellite with pseudo-random coded signals. The GPS System will operate in two simultaneous modes. In one mode, referred to as the clear/acquisition (C/A) mode, the pseudo-random noise (PN) signal is a Gold code sequence that is repeated once every millisecond to enable the position of the receiver responsive to the signals transmitted from one or more of the satellites to be determined to an accuracy better than 100 meters.
E911 requires capability to determine the location of a handset. The location information is required to be passed through a narrow band communication channel. A compressed C/A code can be sent over any cellular communications channel, enabling location of the handset by the receiver. Further, mobile devices require low power consumption, and with compressed messages, lower transmitter and receiver power is used.
It can be seen, then, that there is a need in the art for techniques to compress the C/A code. It can also be seen, then, that there is a need in the art for compressing the C/A code in order to use the C/A code with other remote GPS receiver components. It can also be seen that there is a need in the art to provide the compressed C/A code in a relatively inexpensive manner. It can also be seen that there is an need in the art for low power GPS receivers.